1. Field of the Invention
This invention relates to electrodes for gas tungsten arc welding apparatus.
2. The Prior Art
The gas tungsten arc welding or GTAW apparatus is well-known in the industry and is used for high temperature, precision welding particularly in automated welding systems. The GTAW apparatus contains a nonconsumable electrode and generally operates on direct current power applied to the electrode. The polarity is usually electrode negative (Direct Current Straight Polarity, or DCSP, as commonly referred to in the trade). In operation, an electric arc is formed between the working tip of the electrode and the metal workpiece. The electrode is fabricated from tungsten or an alloy of tungsten in order to withstand the high temperatures involved.
The electrode is concentric within a ceramic gas shield which directs a flow of gas around the electric arc. The arc ionizes the gas through which it passes to create a high temperature plasma for welding. The gas, an inert gas such a helium or argon, serves (1) to create the plasma by being ionized by the electric arc and (2) shields the molten metal at the weld site from reaction with gases in the atmosphere.
Each electrode composition has a given electron density (P) which must be obtained before the electrons will leave the surface of the electrode to form the arc. The area from which the electrons leave the electrode surface is the area of electron emission (A). The relationship between the electron density (P), the area of emission (A) and the current (I) applied to the electrode may be stated as: P = I/A. Since P is a constant value for a given electrode composition, an increase in (I) will correspondingly increase (A). However, each electrode composition also has a maximum continuous operating current above which electrode failure will occur. If the working tip of a cylindrical electrode were the flat, circular face at the end of the electrode, the area of the circular face would be considerably greater than the maximum area of electron emission (A) that could be obtained at the maximum continuous current (I) that could be applied to the electrode. Accordingly, the resulting plasma would be emitted from only a fraction of the area of the circle and would tend to skip erratically over the face of the circle resulting in an unstable plasma and a low repeatability of weld accuracy. It has, therefore, been the accepted practice to localize the area of electron emission for the plasma by reducing the surface area available for electron emission. This is done by shaping the working tip of the electrode as a cone. The conical point may range between blunt and sharp. However, the conical point is generally formed as a sharp, 30.degree. to 120.degree. right circular cone. Although the sharp point is preferred for most applications, a blunt configuration such as a semi-spherical tip is used for AC operation, generally on aluminum, and some DC operations.
Although the electrode is not directly consumed in the welding process, the tip is subjected to erosion, contamination, and wear requiring periodic reworking and, ultimately, replacement of the electrode. Tip erosion appears to result from heating of the apex of the cone and from electron erosion along the grind lines of all but the most carefully prepared tips. Contamination of the working tip results from sputtering of or contact with the weld metal.
A useful discussion of conical tip geometry may be found in Welding Research Supplement, "The Effect of Electrode Welding," W. F. Savage, et. al., November 1965, pages 489-s to page 496-s.
The electron stream or plasma formed from a conical tip results in a "bell" shaped plasma. Specifically, the portion of the plasma emitted from the apex of the cone travels directly to the metal being heated. The electrons emitted from the cone surface adjacent the apex of the cone are emitted perpendicular to the cone surface and are thereafter magnetically attracted by the plasma to the remainder of the electron stream. This phenomenon of being emitted perpendicularly from the surface of the cone and then being drawn to the main body of the electron stream creates the upper curved surface or shoulder of the bell shape. At the metal surface, countercurrently flowing ions in the plasma cause the plasma to flare outwardly thereby creating the expanded periphery or lip of the bell-shaped plasma.
At low amperages, the bell-shaped plasma has poor temperature uniformity and is relatively uncontrollable. This results in erratic heating characteristics of the metal and, consequently, a low rate of repeatability or reproducibility of the welding process. Even at higher amperages it has been found that the bell-shaped plasma is readily deflected by air currents and pivots about the conical tip of the electrode. Furthermore, due to the girth of the bell-shaped plasma, the working tip of the electrode must be extended beyond the confines of the ceramic gas shield to prevent interference between the plasma and the shield.
Increased electrode life and restarts has been obtained by incorporating a coaxial, cylindrical, blind bore in the flat end of an electrode, for example, see U.S. Pat. No. 3,780,259. This patent provides for a coaxial blind bore with the diameter of the bore being between 25 and 50 percent of the electrode so that the face of the working tip is a broad, flat, annular surface. The area of electron emission on this annular surface, using the formula P = I/A, is an area less than the area of the broad annular face. It is not possible for the area of electron emission, (A), to encompass all of the foregoing broad annular face without exceeding the current limitations, (I), of the electrode. A greater number of restarts have been obtained because of the large surface area of the face available for electron emission. As one portion of the face becomes contaminated the area of electron emission merely shifts to an uncontaminated portion of the face. Furthermore, the plasma is also easily deflected by air currents and the like thereby having a tendency to be a very unstable plasma.
In view of the foregoing, it would be a significant advancement in the art of gas tungsten arc welding electrodes to provide an electrode which is configurated to emit a controlled and stable plasma. In particular, it would be advantageous to provide an electrode having an extended life and which emits a more stable and constricted plasma thereby permitting a greater reproducibility of results. Such an invention is disclosed herein.